Performance and effectiveness of adsorbents prepared from
lignocellulosic agro-industrial residues on the abatement of leachate
odor containing ammonia
Thalles Perdigão Lima
Dissertation submitted to the Escola Superior Agrária de Bragança to obtain the Degree of Master in Environmental Technology under the scope of the double diploma
with the Centro Federal de Educação Tecnológica de Minas Gerais (CEFET-MG)
Supervised by Manuel Feliciano Adriana Alves Pereira Wilken
Jose Luis Díaz de Tuesta
Bragança 2020
Performance and effectiveness of adsorbents prepared from
lignocellulosic agro-industrial residues on the abatement of leachate
odor containing ammonia
A dissertation presented to the Escola Superior Agrária de Bragança in partial fulfillment of the requirements for the degree of Master of Science in Environmental Technology in cooperation with the Centro Federal de Educação Tecnológica de Minas Gerais (CEFET-MG) under the double diploma programme.
Supervisors:
Manuel Feliciano (IPB)
Adriana Alves Pereira Wilken (CEFET-MG) Jose Luis Díaz de Tuesta (IPB)
Thank you, Lord, for your great love, care, mercy, and grace. During all difficulties, when I was weak, then you made me strong in you, Jesus. Thank you, mom and dad, for your unconditional love, always there for me even 8.000 km away. I thank my grandmas, cousins, uncles, aunts, and friends for sharing love, encouragement, and prayers. I thank my sister Ingrid for being supportive during the final stage of this work. I am grateful to Marina for being by my side and cheer me up in challenging times.
I want to thank my supervisors Feliciano, Adriana, and Jose, as well as professor Helder, for accepting the challenge of starting a new line of research. I am grateful to professor Feliciano for having seen potential in me and for assisting and trusting. I thank Dr. Jose for knowledge sharing, help, encouragement, and patience. I thank professor Adriana for her contributions to my formation in the past few years and for accepting the challenge of distance supervising. I am also grateful to Dr. Stephen (McCord Environmental Inc.) for supporting me since 2015 and for his valuable contribution to thoroughly reviewing this document.
I am very grateful to my colleagues Adriano, Fernanda, Gabriel, Leonardo Delgado, Leonardo Fürst, and Yago to have supported the work carried out in the laboratories and to Maria João for being always willing to help and host various laboratory tests at the Chemical Process Laboratory. I also would like to thank the volunteers for dedicating time to participate in the sensorial analysis.
I am very appreciative to the Centro Federal de Educação Tecnológica de Minas Gerais for the scholarship and support, in particular the professors of the Departamento de Ciência e Tecnologia Ambiental and staff of the Secretaria de Relações Internacionais. Special thanks to the Portuguese people for your welcome and to the Instituto Politécnico de Bragança, in particular the professors, researchers, and staff, for the outstanding support that made this work to be completed.
I am grateful to the Foundation for Science and Technology (FCT Portugal) for financial support by national funds FCT/MCTES to CIMO (UIDB/00690/2020).
For the Lord gives wisdom; from his mouth come knowledge and understanding. Proverbs 2:6
Pois o Senhor é quem dá sabedoria; de sua boca procedem o conhecimento e o discernimento. Provérbios 2:6
This work aimed to prepare different adsorbent materials (bioadsorbent, pyrochar, hydrochar, and activated carbon), using olive stone and malt bagasse as feedstock and evaluating its performance and effectiveness in the adsorption of ammonia (NH3), deriving from composting leachate. A lab-scale adsorption system was assembled for the adsorption tests. The performance and effectiveness of the adsorbents on NH3 adsorption were evaluated objectively, by chemical analytical measurement, and subjectively by olfactometric assessment using the human sense of smell. The materials' preparation was studied to assess the biomass loss and the carbon released into the liquid phase during the hydrothermal carbonization process. Besides, resultant adsorbents were characterized to study their surface chemistry, elemental analysis, and textural properties. Saturated adsorbents were regenerated using water and subsequently re-used in the adsorption of NH3 coming from the leachate to assess their adsorption capacities after a sorption-desorption cycle. The hydrochar derived from olive stone, prepared by hydrothermal carbonization assisted by sulfuric acid (H2SO4), was found as the best adsorbent for NH3 removal produced in this work since it has the lowest height of mass transfer zone (0.315 - 0.520 cm) and the highest NH3 adsorption capacity (9.445 - 11.421 mg g-1). The bioadsorbent prepared only by milling and drying olive stones was also capable of adsorbing NH3, showing a height of mass transfer zone of 0.484 - 0.565 cm, and an adsorption capacity of 0.975 - 1.455 mg g-1; besides the advantage of being environmentally-sound since it requires low energy expenditure, and no chemicals are used in its preparation. The olfactometric evaluations confirmed that the adsorbents mentioned above, prepared by olive stone, can reduce odor annoyance of the gases derived from leachate. Finally, the regeneration process using water delivered adsorbents capable of being used in one NH3 sorption-desorption cycle, with satisfactory performance (>70% of the mean NH3 adsorption capacity of its respective first-generation adsorbents), leading to increasing the materials' resource-use efficiency.
RESUMO
Este trabalho teve como objetivo preparar diferentes materiais adsorventes (bioadsorvente, pyrochar, hydrochar e carvão ativado), utilizando caroço de azeitona e bagaço de malte como matéria-prima e avaliar seu desempenho e eficácia na adsorção de amoníaco (NH3), proveniente do lixiviado de compostagem. Um sistema de adsorção em escala de laboratório foi montado para os testes de adsorção. O desempenho e eficácia dos adsorventes na adsorção de NH3 foram avaliados objetivamente, por medição analítica química, e subjetivamente, por avaliação olfatométrica usando o sentido do olfato humano. A preparação dos materiais foi estudada para avaliar a perda de biomassa e o carbono liberado na fase líquida durante o processo de carbonização hidrotermal. Além disso, os adsorventes resultantes foram caracterizados para estudar sua química de superfície, sua análise elementar e suas propriedades texturais. Adsorventes saturados foram regenerados com água e posteriormente reutilizados na adsorção de NH3 proveniente do lixiviado para avaliar sua capacidade de adsorção após um ciclo de sorção-dessorção. O hydrochar derivado de caroço de azeitona, preparado por carbonização hidrotermal assistida por ácido sulfúrico (H2SO4), foi apontado como o melhor adsorvente para remoção de NH3 produzido neste trabalho, por apresentar a menor altura de zona de transferência de massa (0,315 - 0,520 cm) e a maior capacidade de adsorção de NH3 (9,445 - 11,421 mg g-1). O bioadsorvente preparado apenas pela moagem e secagem do caroço da azeitona também foi capaz de adsorver NH3, apresentando uma altura de zona de transferência de massa de 0,484 - 0,565 cm e uma capacidade de adsorção de 0,975 - 1,455 mg g-1; além da vantagem de ser ambientalmente adequado, uma vez que requer baixo gasto de energia e nenhum produto químico é utilizado em sua preparação. As avaliações olfatométricas confirmaram que os adsorventes citados acima, preparados a partir de caroço de azeitona, podem reduzir a incomodidade do odor dos gases derivados do lixiviado. Por fim, o processo de regeneração com água forneceu adsorventes capazes de serem utilizados em um ciclo de sorção-dessorção de NH3, com desempenho satisfatório (> 70% da capacidade média de adsorção de NH3 de seus respectivos adsorventes de primeira geração), levando ao aumento da eficiência de utilização dos materiais.
TABLE OF CONTENTS
ABSTRACT ... vi
RESUMO ... vii
List of Tables ... xi
List of Figures ... xiii
List of Abbreviations, Acronyms and Units of Measure ... xvi
1 INTRODUCTION ... 1
1.1 Background ... 1
1.2 Objectives ... 4
1.2.1 General Objective ... 4
1.2.2 Specific Objectives ... 4
1.3 Organization of the thesis ... 5
2 LITERATURE REVIEW ... 7
2.1 Organic waste ... 7
2.1.1 Agro-industrial residues ... 7
2.1.2 Legal framework on solid organic waste ... 11
2.2 Carbonaceous adsorbents ... 16
2.2.1 Carbonaceous materials ... 16
2.2.2 Adsorbent carbonaceous materials ... 17
2.2.3 Adsorbents derived from biomass ... 24
2.2.4 Regeneration of adsorbents ... 30
2.3 Odorous air emissions ... 30
2.3.1 Pollutants and sources of pollution ... 30
2.3.2 Measuring, prevention, and control ... 35
2.3.3 Adsorption of gaseous odorous pollutants ... 36
3 MATERIALS AND METHODS ... 44
3.1 General methodology ... 44
3.2 Materials and equipment ... 45
3.3 Preparation of the adsorbents ... 47
3.3.1 Bioadsorbents ... 47
3.3.2 Pyrochars ... 48
3.3.3 Hydrochars ... 49
3.3.4 Activated carbon ... 51
3.4 Carbon and mass balance ... 51
3.4.1 Biomass loss ... 51
3.4.2 TOC of the liquid effluent obtained by HTC ... 51
3.4.3 Elemental analysis ... 52
3.5 Characterization of the adsorbents ... 52
3.5.1 Surface chemistry characteristics ... 52
3.5.2 Ashes determination ... 53
3.5.3 Textural properties ... 54
3.5.4 Void fraction characteristics ... 55
3.6 Set-up of the lab-scale system ... 56
3.7 NH3 concentration and adsorption ... 59
3.7.1 Evaluation of zero-air and gases derived from leachate ... 60
3.7.2 Objective evaluation of NH3 adsorption ... 60
3.7.3 Subjective evaluation of NH3 adsorption ... 63
3.8 Regeneration of saturated adsorbents ... 67
3.8.1 Experimental procedure ... 67
3.8.2 TOC, conductivity, and pH of the liquid phase ... 69
4 RESULTS AND DISCUSSION ... 70
4.1.1 Biomass loss ... 70
4.1.2 TOC of the liquid effluent obtained by HTC ... 71
4.1.3 Elemental analyses and ashes determination ... 72
4.2 NH3 concentration and adsorption ... 73
4.2.1 NH3 concentration in zero-air ... 73
4.2.2 NH3 concentration in feed stream from leachate ... 74
4.2.3 Chemical analysis of NH3 adsorption ... 76
4.2.4 Olfactometric analysis of NH3 adsorption ... 79
4.3 Characterization of the adsorbents ... 82
4.3.1 Surface chemistry characteristics ... 82
4.3.2 Textural properties ... 84
4.3.3 Void fraction characteristics ... 86
4.4 Regeneration of saturated adsorbents ... 87
4.4.1 NH3 adsorption with regenerated samples... 87
4.4.2 Biomass loss ... 90
4.4.3 TOC, conductivity, and pH of the liquid phase ... 90
5 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ... 93
5.1 Conclusions ... 93
5.2 Recommendations for future work ... 94
LIST OF REFERENCES ... 96
APPENDIX A ... 108
APPENDIX B ... 109
APPENDIX C ... 110
List of Tables
Table 1. Proximate and ultimate analysis of various agricultural waste (Yahya et al., 2015). ... 8 Table 2. Lignocellulosic compositions of agricultural residues. Based on Razi et al. (2018), Yahya et al. (2015), Blanco López et al. (2002), Cagnon et al. (2009) and González et al. (2009). ... 9 Table 3. Surface area values (m2.g-1) for physically activated carbons (ACs) obtained from different lignocellulosic precursors. Adapted from González-García (2018) ... 25 Table 4. Surface area values (m2.g-1) for chemically activated carbons (ACs) obtained from different lignocellulosic precursors. Adapted from González-García (2018) ... 27 Table 5. Characteristics and detection thresholds of the main odoriferous compounds. 34 Table 6. Types of adsorbent, process of preparation, feedstock, and sample label of the adsorbents prepared. ... 47 Table 7. Sample labeling and regeneration operating conditions. ... 68 Table 8. Biomass loss (B.L) of samples prepared in this work. ... 70 Table 9. Total Organic Carbon (TOC) of the liquid phase resulting from the H2SO 4-assisted HTC processes. ... 71 Table 10. Elemental composition of the adsorbents. ... 72 Table 11. Values of the mass of adsorbent (m) and height of the adsorption bed (Z), and the results of NH3 inlet concentration (C0), breakthrough time (tb), stoichiometric time (tsto), saturation time (tsat), height of mass transfer zone (HMTZ), and dynamic adsorption capacity (qa). ... 77 Table 12. Acidity and basicity of the surface of fresh and saturated first-generation adsorbents. ... 83
Table 13. Textural properties of the adsorbents: specific surface area (SBET), external surface area (Sext), micropore surface area (Smic), micropore volume (Vmic), total pore volume (Vtotal), and average pore diameter (Wmic). ... 85 Table 14. Void fraction characteristics determination: volume of adsorbent material (Vm), volume of distilled water (Vw), total volume (Vt). void volume (Vvoid), and void fraction (Voidf). ... 86 Table 15. Values of the mass of adsorbent regenerated (m), regeneration temperature (T), volume of ultrapure water (VUPw), mass of regenerated adsorbent in the fixed-bed column (FBm), height of the adsorption bed (Z), and the results of NH3 inlet concentration (C0), breakthrough time (tb), stoichiometric time (tsto), saturation time (tsat), height of mass transfer zone (HMTZ), and dynamic adsorption capacity (qa). ... 89 Table 16. Parameters of the regeneration processes and biomass loss: regeneration temperature (T), volume of ultrapure water (VUPw), initial and final mass of adsorbent, and biomass loss (B.L). ... 90 Table 17. Parameters of the regeneration processes and liquid phase characteristics: mass of adsorbent before regeneration (m), temperature (T), volume of ultrapure water (VUPw), TOC, and conductivity and pH of the liquid phase (before and after the regeneration). 92
List of Figures
Figure 1. Waste hierarchy (European Union, 2008b). ... 15
Figure 2. Representation of a carbonaceous adsorbent: (a) amorphous form; (b) porous structure with adsorbate; (c) aromatic clusters. Based on Celzard et al. (2007). ... 18
Figure 3. Schematic of pyrochar structure development under many temperature ranges (Lehmann et al., 2009). ... 20
Figure 4. Hydrochar formation by hydrothermal carbonization (HTC) (Jain et al., 2016). ... 21
Figure 5. Preparation of AC by HTC, followed by chemical activation (Jain et al., 2016). ... 24
Figure 6. Ideal breakthrough curve: fixed-bed dynamic behavior in which gas is injected at a constant concentration of a particular pollutant at the column entrance. Adapted from Ang et al. (2020) and Tan et al. (2017). ... 38
Figure 7. Schematic diagram of the stages of this work. ... 45
Figure 8. The feedstock (a) OS and (b) MB. ... 45
Figure 9. The centrifugal mill and ring sieve with trapezoid holes used to mill the olive stones. ... 48
Figure 10.The bioadsorbents (a) OS-M and (b)MB prepared in this work. ... 48
Figure 11. The horizontal tube furnace used to perform the pyrolysis... 49
Figure 12. The pyrochars (a) OS-M-P and (b) MB-P prepared in this work. ... 49
Figure 13. The high-pressure batch reactors used to perform the H2SO4-assisted HTC. 50 Figure 14. The hydrochars (a) OS-M-HTC, and (b) MB-HTC prepared in this work. .. 50
Figure 15. Filtration and washing of the HTC’s solid product. ... 50
Figure 17. The flasks containing the mixture of adsorbent and solution placed in the orbital shaker. ... 52 Figure 18. The furnace used in the ash analysis. ... 54 Figure 19. Main types of physisorption isotherms established by IUPAC (Thommes et al., 2015). ... 55 Figure 20. Schematic of the set-up of the lab-scale system for evaluating: (a) zero-air (za), (b)leachate off-gases, and (c) the performance and effectiveness of the adsorbents. .... 58 Figure 21. Photo of the lab-scale system assembled. In detail: source of odor (SO), upstream gas for subjective evaluation (ugs), fixed-bed (FB) column, downstream gas for objective evaluation (dgo), downstream gas for subjective evaluation (dgs), and multi-gas analyzer (MGA). ... 59 Figure 22. Monitoring station containing the air temperature sensor and the data logger. ... 60 Figure 23. Schematic of the fixed-bed column: (1) cap, (2) glass wool, (3) mesh, and (4) adsorbent. ... 61 Figure 24. Illustration of the integration calculation using the Peak Analyzer tool in a real breakthrough curve obtained in an adsorption test performed in this work. ... 63 Figure 25. Pictures of sampling and storing steps: (a) empty sample bag, (b) sample being taken from the adsorption test, and (c) bags filled with samples stored. ... 64 Figure 26. Schematic representation of downstream samples on the breakthrough curve. SEClean - at maximum removal capacity; SEBreak - between breakthrough and stoichiometric times; SEStoic - between stoichiometric and saturation times; and SESatur - right after saturation... 65 Figure 27. Odor (a) intensity and (b) hedonic tone scales. Based on (VDI, 1992a). ... 66 Figure 28. Set-up of the olfactometric analysis: (a) olfactometer and (b) the sniffing ports in detail. ... 66
Figure 30. Concentrations of NH3 in the zero-air (za). ... 74 Figure 31. Tests that lasted 8 h: (a) temperature during the tests and (b) concentration of NH3 emitted from leachate... 75 Figure 32. Tests that lasted 24 h: temperature during the tests and concentration of NH3 emitted from leachate. (a) Test 1, and (b) Test 2. ... 76 Figure 33. Normalized breakthrough curves of the adsorption tests performed in this work. ... 77 Figure 34. Evaluation of odor hedonic tone of samples derived from adsorption tests using (a) OS-M and (b) OS-M-HTC. ... 80 Figure 35. Evaluation of odor intensity of samples derived from adsorption tests using (a) OS-M and (b) OS-M-HTC. ... 81 Figure 36. N2 adsorption-desorption isotherms of the adsorbents at 77 K. ... 85 Figure 37. Normalized breakthrough curves of adsorption tests using regenerated (R_X_OS-M and R_X_OS-M-HTC) and first-generation (OS-M and OS-M-HTC) samples. (A) bioadsorbents and (B) hydrochars. ... 87 Figure 38. Possible improvement of the stages of this work after future studies. ... 95
List of Abbreviations, Acronyms and Units of Measure Abbreviations and Acronyms
AC activated carbon B.L biomass loss
dgo downstream gas for objective evaluation dgs downstream gas for subjective evaluation eo effluent outlet
FB fixed-bed
FC flow control
FM flow meter
HTC hydrothermal carbonization
IUPAC International Union of Pure and Applied Chemistry
MB malt bagasse
MGA multi-gas analyzer MTZ mass transfer zone
OFGs oxygenated functional groups
og odorous gas
OS olive stone
PM10 airborne particles of equivalent aerodynamic diameter less than 10 m PM2.5 airborne particles of equivalent aerodynamic diameter less than 2.5 m SDGs United Nations Sustainable Development Goals
SEClean sample taken downstream the FB, at maximum removal capacity
SEBreak sample taken downstream the FB, between breakthrough and stoichiometric points SEFeed gas sample taken upstream the FB
SESatur sample taken downstream the FB, right after saturation point
SO source of odor
SP1 sniffing port providing neutral air SP2 sniffing port providing odor stimulus TOC Total Organic Carbon
ugs upstream gas for subjective evaluation UPw ultrapure water
VOCs volatile organic compounds Voidf void fraction
WHO World Health Organization
za zero-air
ZAG zero-air generator
Units of Measure
Co initial concentration of adsorbate [ppm]
FBm mass of regenerated adsorbent in the fixed-bed column [g] HMTZ height of mass transfer zone [cm]
m mass of adsorbent [g]
qa dynamic adsorption capacity [mg g-1] Q gas flow rate [L min-1]
SBET BET specific surface area [m2] Sext external surface area [m2 g-1] Smic micropore surface area [m2 g-1] tb breakthrough time [min] tsat saturation time [min]
tsto stoichiometric time [min]
Vm volume of adsorbent material [mL] Vw volume of distilled water [mL] Vmic micropore volume [mm3 g-1] Vtotal total pore volume [mm3 g-1] Vvoid volume of voids in the bed [mL] Wc weight of the crucible [g]
Wci weight of the crucible + weight of inorganic material [g] Wm weight of adsorbent material [g]
Wmic average pore diameter [nm] Z height of the adsorption bed [cm]
1 INTRODUCTION
1.1 Background
Odorous pollutants may result directly or indirectly from human activities (e.g., composting plants, landfills, and wastewater treatment plants). They may cause adverse effects, including various undesirable reactions, ranging from annoyance to documented health effects. In residences and workplaces exposed to odors resulting from gaseous emissions, even though the affected individuals may not immediately appear diseased or infirm, there certainly is not an atmosphere of complete mental, social, or physical well-being (Nicell, 2009).
Despite contributing to proper waste management, landfill facilities and compost plants typically are sources of odor pollution (Rincón et al., 2019). A study carried out by Cheng et al. (2019) showed that both waste treatment facilities mentioned above have NH3 as one of the most critical offensive odorants that should be considered on health risk assessment.
Agro-industrial solid waste is mostly composed of organic matter, known as biomass, that comes from plants. The biomass stores chemical energy in the form of carbohydrates (Sansaniwal et al., 2017). It is the only renewable source of carbon that can be transformed into a solid, liquid, and gaseous products through various conversion processes (Mohamed et al., 2010). Among the appropriate destinations of the biomass residue produced in agro-industrial activities is converting this inexhaustible, low-cost, and non-hazardous biomass into carbonaceous materials (Mohtashami et al., 2018).
Research is underway in the field of agricultural waste valorization as potential adsorbent materials for pollution control since they have a porous structure and a macromolecular matrix that consists of polymer chains which contain numerous polyvalent functional groups (such as carboxylic and hydroxylic reactive groups) (Calero et al., 2010). Efforts are being made to achieve sustainable production of carbonaceous materials derived from biomass. Sustainable production of these adsorbents requires proper management of biomass, water, energy, chemicals, and appropriate residuals management (Daramola & Ayeni, 2020).
Olive-based products are primary elements in the EU’s southern countries’ agricultural economy, with about 5 million hectares of plantations and more than €7 billion in production value every year (Rossi, 2017). The EU accounts for 70 to 75 % of the world production of olive oil. More than half of the olive plantation areas in the EU are located in Spain, and most of it is used for olive oil production (Rossi, 2017). The main byproduct generated during the processing of olives for oil production is a residue named olive cake, consisting of a mixture of crushed stones (also known as pits), seeds, peel, and pulp (Sanginés et al., 2015). The production of olives for oil and table use in the EU was about 10.5 million tons in the 2018-2019 harvesting year (Eurostat, 2020). It is estimated that olive stone (OS) ranges from 8 to 15% of the weight of the olives (Pattara et al., 2010), so that about 0.8 to 1.6 million tons of OS may have been generated during the processing of olives in the EU in 2018-2019.
Brazil is the third-largest beer producer globally, after China and EUA, and has produced 13.3 billion liters of the product in 2016 (SINDICERV, 2020). Malte bagasse (MB) is considered the most crucial residue of the beer production process (Mello & Mali, 2014). That residue is generated in the cereals’ filtration used in the brewing process (Nadolny et al., 2020). It is estimated that approximately 17 to 20 kg of MB is generated to produce 100 liters of beer (Franciski et al., 2018), so that about 2.3 to 2.7 million tons of bagasse may have been generated during beer production in Brazil in 2016.
The valorization of agro-industrial biomass as a precursor of adsorbents is a practical manner to increase resource-use efficiency. It keeps natural resources in use for as long as possible, which is the principle of the circular economy (Baldikova et al., 2019). Also, it may contribute to reducing the use of non-renewable resources in the commercial production of adsorbents. Bioadsorbent, pyrochar, hydrochar, and activated carbon (AC) are types of adsorbent materials synthesized from biomass, thus producing environmentally sound adsorbents for pollutant removal (Chen et al., 2017; Eslami et al., 2018).
Bioadsorbent consists of low-cost organic matter with a natural affinity for inorganic and organic pollutants (Fomina & Gadd, 2014; Safarik et al., 2018). Pyrochar is the solid char remaining from the organic matter’s thermal decomposition under an inert atmosphere, a high-carbon material with good porosity and surface area (Ioannidou & Zabaniotou, 2007). The thermal treatment removes the moisture and the volatile matter
contents of the biomass (Lehmann & Joseph, 2009; Mohamed et al., 2010). Hydrochar is a material obtained by hydrothermal carbonization (HTC), a thermo-chemical process, which uses water, heat, and high pressure to convert biomass into carbonaceous materials through fractionation of the feedstock (Daramola et al., 2020; Jain et al., 2016; Ok et al., 2016). According to the International Union of Pure and Applied Chemistry (IUPAC), AC is “a porous carbon material, a char which has been subjected to reaction with gases, sometimes with the addition of chemicals, before, during or after carbonization in order to increase its adsorptive properties” (IUPAC, 1997).
Agricultural biomass waste could be the basis of a low-cost NH3 adsorbent, allowing the recycling of residues and the abatement of air pollutants simultaneously (Kastner et al., 2009). In the literature, many studies have focused on producing adsorbents derived from biomass and their assessment of adsorption tests of gaseous pollutants derived from commercial bottles. This work presents a different approach focused on bioresource technology to produce adsorbents and their application in emission control of an odor’s actual source.
Based on the three fundamental pillars of sustainability, this work seeks to improve environmental, economic, and social spheres: organic matter recycling, air pollution abatement, resource-use efficiency, environmentally sound technology development, health and well-being, and safety. As part of the 2030 Agenda for Sustainable Development, the United Nations Sustainable Development Goals (SDGs) consists of seventeen global goals for the next eleven years in areas of critical importance for humanity and the planet (United Nations, 2015). This work is directly aligned with four SDGs, as shown below.
Goal 3 (Good Health and Well-being) has included targets for air pollution prevention and control to ensure healthy lives and promote the well-being of all. The definition of “health” given by the World Health Organization (WHO) is “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity”. Given the above, the importance of controlling odorous gas emissions is noticeable because even the odor annoyance can affect well-being.
Goal 9 (Industries, Innovation, and Infrastructure) includes targets for increased resource-use efficiency and greater adoption of clean and environmentally sound technologies and industrial processes. Agro-industrial biomass’s valorization as a
precursor of adsorbents is a practical manner to increase resource-use efficiency, producing an environmentally sound technology for pollutant removal. Additionally, re-using this recovered product reduces non-renewable material on commercial adsorbents’ production.
Goal 11 (Sustainable Cities and Communities) includes targets for reducing adverse per capita environmental impact of cities and communities by paying particular attention to air quality and municipal and other waste management. Once again, the importance of controlling gas emission is evident, including the odorous ones, and besides the proper management of agro-industrial waste, not to mention the agricultural biomass. Goal 12 (Responsible Consumption and Production) includes targets for the environmentally sound management of chemicals and all wastes throughout their life cycle by reducing their release to air, water, and soil to minimize their adverse impacts on human health and the environment. This goal also includes targets for substantial waste generation abatement through prevention, reduction, recycling, and re-use. Reiteratively recycling organic biomass by converting it into new materials reduces waste released into the environment, reducing its adverse impacts.
1.2 Objectives
1.2.1 General Objective
This thesis aims at preparing adsorbent materials using olive stone and malt bagasse as feedstock and evaluates its performance and effectiveness in the adsorption of NH3, deriving from leachate originated from a composting line of mechanical and biological treatment of municipal organic wastes, by using an experimental system developed for this purpose.
1.2.2 Specific Objectives
The specific objectives of this research are to:
I. provide a literature review about agro-industrial residues, carbonaceous adsorbents and odorous air emissions;
II. prepare adsorbent materials using agro-industrial organic residues as feedstock;
III. characterize the materials produced; IV. set-up an experimental adsorption system;
V. evaluate the odorous pollutant NH3, generated from zero-air generator and leachate, regarding emission rates;
VI. evaluate the performance and effectiveness of the materials in the adsorption of NH3 by chemical analytical measurement;
VII. prepare a low-cost olfactometer to access the olfactory analysis;
VIII. evaluate the performance and effectiveness of the materials in the adsorption of NH3 by olfactometric assessment;
IX. evaluate the regeneration of adsorbents saturated with NH3 by an environmentally sound process.
1.3 Organization of the thesis
Chapter 1 introduces the thesis. Firstly, it presents the background of the central themes, including the emission and negative impacts of odorous pollutants, agro-industrial organic waste production and valorization, the adsorption of odorous pollutants, and the sustainable approach of this work confirmed by its alignment with the United Nations Sustainable Development Goals (SDGs). The background is followed by the presentation of the general and specific objectives of this work.
Chapter 2 presents relevant literature regarding organic waste, carbonaceous adsorbents, and odorous emissions. The first topic presents the characteristics of lignocellulosic residues, its different applications, and the Brazilian and Portuguese legal frameworks on this theme. The second topic presents the various applications of carbonaceous materials, the types and characteristics of adsorbents derived from biomass, and the regeneration processes for re-use. Lastly, the third topic presents the pollutants and sources of odor, the prevention and methods of measuring and controlling odorous emissions, the methods of adsorption of odorous pollutants, and the Brazilian and Portuguese legal frameworks on this theme.
Chapter 3 covers the detailed methodology of this work, which was developed based on the literature. The materials and equipment used are presented. The methods of preparation of four types of adsorbents are described. The methods to determine the biomass loss, the total organic carbon in the liquid effluent, and elemental analysis of the adsorbents are presented. The methods of characterization of the adsorbents regarding surface chemistry characteristics, ashes, textural properties, and void fraction are presented. The setting-up of the lab-scale system assembled to run adsorption tests is explained. The evaluation methods of zero-air and leachate off-gases, and the objective and subjective evaluation of NH3 adsorption are detailed. Lastly, it presents the regeneration method of the adsorbents statured with NH3 using water and the characteristics of this process’s liquid effluent.
Chapter 4 presents the results obtained after the development of the steps explained in Chapter 3.
2 LITERATURE REVIEW
This literature review chapter begins with organic waste, presenting the lignocellulosic residues’ characteristics, its different applications, and the Brazilian and Portuguese legal frameworks on this theme. The following topic presents the various applications of carbonaceous materials, the types and characteristics of adsorbents derived from biomass, and the regeneration processes for re-use. The final topic presents the pollutants and sources of odor, the prevention and methods of measuring and controlling odorous emissions, specifies the adsorption method, and the Brazilian and Portuguese legal frameworks on this theme.
2.1 Organic waste
2.1.1 Agro-industrial residues
The European Commission defines waste as “any substance, material, or object which the holder discards or intends or is required to discard” (European Union, 2008b), and therefore, is no longer useful for the holder (Islas et al., 2019). Solid waste is any waste in the solid and semi-solid state resulting from industrial, domestic, hospital, commercial, agricultural, or municipal activities (ABNT, 2004). The waste may be divided into two main groups: organic waste (composed of organic matter); and non-organic waste (composed essentially of innon-organic matter).
Agro-industrial solid waste is mostly composed of organic matter, known as biomass, that comes from plants. The biomass stores chemical energy in carbohydrates by combining solar energy and carbon dioxide using the photosynthesis process (Sansaniwal et al., 2017). It is considered the only renewable source of carbon that can be transformed into a solid, liquid, or gaseous products through diverse processes (Mohamed et al., 2010).
The OS, MB, and other agricultural organic waste are considered non-hazardous, which means that, because of its physical, chemical, and infectious properties, it does not pose risks to human health, either to the environment when properly managed (ABNT, 2004). However, many developing countries produce immense biomass waste and
destroy or burn them inefficiently, causing pollution of the environment (Bhatnagar et al., 2016).
Most agricultural wastes are considered carbon-containing lignocellulosic materials (Uçar et al., 2009). The high amounts of carbon (C), oxygen (O), and hydrogen (H) present in this type of waste are related to the three major structural polymers present in lignocellulosic feedstock: cellulose, hemicellulose, and lignin (González-García, 2018). Table 1 presents the composition of various agricultural wastes, by proximate and ultimate analysis, based on a scientific literature review carried out by Yahya, Al-Qodah, & Ngah (2015).
Table 1. Proximate and ultimate analysis of various agricultural waste (Yahya et al., 2015).
Moisture Ash Volatiles C H N S O
Palm shell 7.96 1.10 72.47 50.01 6.90 1.90 0.00 41.00 Palm stem 6.06 4.02 72.39 45.56 5.91 0.82 - 47.71 Grape stalk 15.69 10.16 51.08 46.14 5.74 0.37 0.00 36.60 Bamboo - 3.90 80.60 43.80 6.60 0.40 0.00 -Coconut shell 8.21 0.10 73.09 48.63 6.51 0.14 0.08 44.64 Olive mill <5.0 <1.0 - 45.64 6.31 1.42 - -Almond shell 10.00 0.60 80.30 50.50 6.60 0.20 0.01 42.69 Wallnut shell 11.00 1.30 71.80 45.10 6.00 0.30 0.00 48.60 Almond tree pruning 10.60 1.20 72.20 51.30 6.50 0.80 0.04 41.36 Olive stone 10.40 1.40 74.40 44.80 6.00 0.10 0.01 49.09
Bamboo 2.44 6.51 69.63 45.53 4.61 0.22 -
-Durian shell 11.27 4.84 - 39.30 5.90 1.00 0.06 53.74
Chinese fir sawdust 4.88 0.32 79.92 48.95 6.54 0.11 0.00 39.20 Banana empty fruit bunch 5.21 15.73 78.83 41.75 5.10 1.23 0.18 51.73 Delonix regia fruit pods 0.22 2.80 92.03 34.22 4.50 1.94 0.42 58.91
Corn cob 4.30 0.90 78.70 46.80 6.00 0.90 - 46.30 Pomegranate seed 5.38 1.83 78.71 49.65 7.54 4.03 0.65 38.13 Birch 4.40 0.18 - 48.40 5.60 0.20 - 45.80 Salix 7.30 0.75 - 48.80 6.20 1.00 - 43.40 Sugarcane bagasse 6.20 0.90 - 47.30 6.20 0.30 - 46.20 Wheat straw 3.30 3.23 - 46.50 6.30 0.90 - 46.30 Bagasse - 6.20 83.30 41.55 5.55 0.03 - 52.86 Rice husk - 16.70 67.50 36.52 4.82 0.86 - 41.10 Cassava peel 11.40 0.30 59.40 59.31 9.78 2.06 0.11 28.74 Rice stalk 14.17 14.93 66.33 40.79 7.66 1.17 0.49 49.89 Woody birch 6.60 0.20 81.20 48.40 5.60 0.20 - 45.80
Proximate analysis (% w/w) Ultimate analysis (% w/w) Agricultural waste
In the case of OS, elemental analysis has shown the composition weight percentages (%wt): 43.1 – 52.34 C; 5.9 – 7.11 H; 0.03 - 1.0 nitrogen (N); 0.01 – 0.8 sulfur (S); 40.47 – 49.1 O, and 0.37 – 4.4 ash (Cagnon et al., 2009; Ghouma et al., 2015; González et al., 2009; Martín-Lara et al., 2013). The amount of elemental C of OS is considered one of the highest among various stone fruits (Saleem et al., 2019). On the other hand, MB elemental analysis has shown the %wt: 46.84 C; 8.18 H; 3.86 N; 0.38 S; 40.74 O, and 2.78 ash (Franciski et al., 2018; Mello et al., 2014).
All three major structural polymers (cellulose, hemicellulose, and lignin) play an essential role in the porosity of the chars and ACs. The lignin has been identified as the main component responsible for high values of the external area and good porosity characteristics (Cagnon et al., 2009; González et al., 2009; Williams & Besler, 1993). Table 2 shows the lignocellulosic composition of a variety of agricultural wastes based on scientific literature reviews.
Table 2. Lignocellulosic compositions of agricultural residues.
Based on Razi et al. (2018), Yahya et al. (2015), Blanco López et al. (2002), Cagnon et al. (2009) and González et al. (2009).
Agricultural waste Lignin (%) Cellulose (%) Hemicellulose (%)
Almond shell 24.8 32.5 25.5
Almond tree pruning 25.0 33.7 20.1
Apple pulp 21.0 16.0 16.0
Banana empty fruit bunch 19.06 8.3 21.23
Cassava waste 2.2 18.47 6.01
Cocoa pods 0.95 41.92 35.26
Coconut husk 3.54 0.52 23.7
Coconut shell 30.1-50.0 14.0-19.8 32.0-68.7
Flamboyant fruit pod 23.36 13.9 24.13
Kola nut pod 21.29 38.72 40.41
Lemon waste 7.22 18.49 6.07
Olive stone 32.6-50.45 11.82-30.8 15-24.16
Palm shell 53.4 29.0 47.7
Plantain peel (ripe) 1.63-1.75 13.87 15.07 Plantain peels (unripe) 1.75 10.15 11.38
Plum pulp 39.0 6.5 14.5
Plum stone 49.0 23.0 20.0
Table 2. Lignocellulosic compositions of agricultural residues.
Based on Razi et al. (2018), Yahya et al. (2015), Blanco López et al. (2002), Cagnon et al. (2009) and González et al. (2009).
Agricultural waste Lignin (%) Cellulose (%) Hemicellulose (%)
Pomegranate seed 39.67 26.98 25.52
Soft wood 30.5 36.0 18.5
Sugarcane bagasse 18.0-24.0 42.2-55.0 20.0-36.0
Walnut shell 18.2 40.1 20.7
OS is composed predominantly of lignin with approximately 32.6 – 50.45% lignin; 11.82 – 30.8 % cellulose; and 15.0 – 24.16% hemicellulose, thus making it ideal as precursors for adsorbents (Blanco López et al., 2002; Cagnon et al., 2009; Razi et al., 2018; Saleem et al., 2019; Yahya et al., 2015). On the other hand, MB is approximately 24.05 –26.13% lignin; 11.35 – 12.29 % cellulose; and 23.41 – 28.97% hemicellulose, also ideal as a precursor for adsorbents (Mello et al., 2014).
Several lignocellulosic wastes are potential feedstock for bioenergy production, as liquid biofuels (via pyrolysis) or solid fuels (biomass pellets) (Volpe et al., 2018). Biofuel can be produced from several food crops, including grains (maize, sorghum, and wheat), sugar crops (sugarcane, sugar beet), starch crops (cassava), oilseed crops (canola/rape, soybean, and oil palm), and olive crops (olive stones) (Bordonal et al., 2018). Despite savings in greenhouse gases emissions, by substituting fossil fuels, the combustion of biomass usually results in other atmospheric pollutants that may also be detrimental to the environment and human health (e.g., volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxides (NOx), and particulates (soot and ash)) (Sanginés et al., 2015).
Thanks to the development of various waste recycling technologies, an increasing number of technological solutions for agricultural waste are emerging (Spalvins et al., 2018). Among the technological approaches, it is the extraction of high value-added products such as enzymes, single-cell oil, building block chemicals, single-cell protein (Spalvins et al., 2018), nanocellulose (Rajinipriya et al., 2018); and supercritical carbon dioxide extraction of waxes from waste (Al Bulushi et al., 2018). In the construction sector, oil palm shells have been found useful as coarse aggregate in structural concrete (Mannan & Ganapathy, 2004). Various agricultural waste has been found helpful in
developing bricks (Raut et al., 2011). Treuer et al. (2017) observed a good synergism between orange peels and pulp, tropical forest restoration, and carbon sequestration in Costa Rica.
Adsorption has been successfully employed to remove inorganic and organic pollutants in the environment in general (Dai et al., 2018). The use of biomass waste for this purpose reduces the environmental waste burden and achieves the effect of treating pollution with waste (Y. Y. Huang, 2017).
2.1.2 Legal framework on solid organic waste
2.1.2.1 Brazilian Approach
The Brazilian Política Nacional de Resíduos Sólidos (National Policy on Solid Waste), established by Law nº 12.305, of 02 August 2010 (Brasil, 2010), provides guidelines on integrated management of solid waste, the responsibilities of the waste producers and public authorities, and the applicable economic instruments. The approval of Law nº 12.305 has initiated joint work between the public spheres, the productive sector, and civil society to search for solutions to solid waste problems.
In conformity with Article 6, the sustainable development and the recognition of reusable and recyclable solid waste as an economic good of a social value stand out as principles of the abovementioned Policy. In accordance with Article 7, essential objectives of this law are (II) the non-generation, reduction, re-use, recycling, and treatment of solid waste, as well as environmentally appropriate final disposal of waste; and (IV) the adoption, development, and improvement of clean technologies as a means of minimizing negative environmental impacts.
Article 8 brought among the policy’s essential instruments the scientific and technological research. Article 14 presented a critical plan, the Plano Nacional de Resíduos Sólidos (National Plan on Solid Waste), which covers the many types of waste generated in the country, possible waste management alternatives, as well as corresponding goals and actions for various scenarios.
Law nº 12.305 mentions the term “organic waste” only once in Article 36 as a shared responsibility (V) implementing a composting system for organic waste and
articulating ways of using the produced compost. Although the above exposed, based on the definition of “recycle” given by Article 3 as “waste transformation processes that involve chemical, physical-chemical or biological changes, in view of producing inputs or new products”, recycling of organic waste should not be only by biological processes but also by chemical and physical-chemical processes.
It was possible to verify many Brazilian legal texts regulating the production and use of biofertilizers derived from organic waste (e.g., Law nº 6.894/80, Decree nº 4.954/04); however, no legal documents discuss other applications of recycled organic waste.
Regarding the local legal framework, the Política Estadual de Resíduos Sólidos (State Policy on Solid Waste) of Minas Gerais, established by Law nº 18.031 (Minas Gerais, 2009), of 12 January 2009, provides guidelines on integrated management of solid waste in the state of Minas Gerais. Article 51 mentions organic waste just twice. A public financial incentive is offered for initiatives that use municipal and rural organic waste for energy production and rural organic waste recovery from intensive livestock. Law nº 21.557 (Minas Gerais, 2014), of 22 December 2014, adds to Law nº 18.031 the prohibition of using incineration technology as the final destination of municipal waste, including the organic fraction.
It was observed that, as well as the federal legal documents, the local legislation in Minas Gerais does not cover the incentive or application of technologies for recycling organic waste.
2.1.2.2 European and Portuguese approach
Directive 2008/98/EC (European Union, 2008b) of the European Parliament and the European Council, of 19 November 2008, also called Waste Framework Directive, defines key concepts such as waste, recovery, and disposal. It also establishes the requirements for managing waste, including the obligation for an establishment or undertaking carrying out waste management operations to obtain a permit or be registered and an obligation for the Member States to formulate waste management plans. Those plans shall, alone or in combination, cover the entire geographical territory of the EU’s States.
The directive mentioned above brings important definitions in Article 3. First, “treatment” is defined as “recovery or disposal operations, including preparation before recovery or disposal”. This definition brings the word “recovery”, which is defined as “any operation the principal result of which is waste serving a useful purpose by replacing other materials which would otherwise have been used to fulfill a particular function, or waste being prepared to fulfill that function, in the plant or the wider economy”. Annex II sets out a non-exhaustive list of recovery operations, among which is the R3 – Recycling/reclamation of organic substances which are not used as solvents (including composting and other biological transformation processes). It includes preparing for re-use, gasification, and pyrolysis using the components as chemicals and recovery of organic materials in the form of backfilling. The term “recycling” pointed before is defined as “any recovery operation by which waste materials are reprocessed into products, materials or substances whether for the original or other purposes. Based on the definitions and processes presented above, reprocessing of organic material, which includes processing and use of agricultural biomass waste as a precursor of adsorbents, it is a recycling treatment, but the energy recovery and the reprocessing into materials that are to be used as fuels or for backfilling are not.
Nonetheless, Article 2 excludes from the scope of the Directive 2008/98/EC “…straw and other natural non-hazardous agricultural or forestry material used in farming, forestry, or to produce energy from such biomass through processes or methods which do not harm the environment or endanger human health”. As Annex III does not cover the agricultural biomass waste, it is characterized as natural non-hazardous waste. Despite the exclusion of agricultural waste from its scope, the Directive 2008/98/EC includes food processing plants as “bio-waste”, according to its Article 3. It also demonstrates that promoting the separate collection and adequate treatment of bio-waste is fundamental to produce environmentally safe compost and other bio-bio-waste-based materials.
There are no specific legal EU documents regarding agro-industrial organic waste production and destination. However, the European Waste List, established by Decision 2014/955/UE, of 18 December 2014 (European Union, 2014), presents the list of waste referred to in Article 7 of Directive 2008/98/CE. It is a harmonized list of waste that considers the origin and composition of the waste. The waste from agriculture,
horticulture, aquaculture, forestry, hunting, and fishing is defined by the four-digit code 0201. OS and MB are included in the abovementioned group as a plan-tissue waste, represented by the six-digit code 020103.
Directive 2018/851/EC (European Union, 2018) of the European Parliament and the European Council, of 30 May 2018, introduced an amendment to Directive 2008/98/CE on solid waste. The first topic of Directive 2018/851/EC presents the need to improve and transform “waste management” into “sustainable material management” to protect, preserve, and improve the environment’s quality. Another critical point of this amendment concerns improving resource use, valuing waste, reducing dependence on imported raw materials, and facilitating the transition to more sustainable material management and a circular economy model. The amendment takes measures on sustainable production and consumption by focusing on the whole life cycle of products to preserve resources and closes the loop to make the economy truly circular.
Considering the circular economy approach’s inclusion as an essential matter in waste management, recycling materials to use them for technological applications can improve the environment’s quality. The use of recycled agricultural biomass as adsorbents of pollutants contributes twice to the environmental quality improvement once it reinserts the material in the process, as proposed by the circular economy, and simultaneously contributes to pollution control.
Portuguese Decree-Law nº 73/2011(Portugal, 2011), of 17 July 2011, introduced the third amendment to Decree-Law nº 178/2006, of 05 September 2006, with changes in the general regulation of waste management and transposed Directive 2008/98/CE. Under the Member States’ obligation to draw up waste management plans, imposed by Directive 2008/98/EC, Portugal elaborated the Plano Nacional de Gestão de Resíduos (National Solid Waste Management Plan) 2014-2020. This plan established national strategic guidelines for waste prevention policy and waste management and the guiding rules ensuring the coherence of specific waste management instruments. One topic covered in this plan is the Green Economy, which has policies that point to a circular economy. The circular economy proposes that the waste generated in a production/consumption process should recirculate as input in the same or another process.
Complementarily Portugal also established specific strategic plans according to the source of waste. The Plano Estratégico dos Resíduos Agrícolas (Strategic Plan on
Agricultural Solid Waste) should be the one to cover plans for agricultural waste; however, it is still being developed. Thus, the Plano Estratégico dos Resíduos Industriais (Strategic Plan on Industrial Solid Waste), approved by Decree-Law nº 89/2002 (Portugal, 2002), includes agricultural waste in its scope but does not mention agro-industrial solid waste not once.
According to the European Commission (2018), waste management means “the collection, transport, recovery (including sorting), and disposal of waste, including the supervision of such operations and the after-care of disposal sites, and actions taken as a dealer or broker”. It also mentions it as an essential process that should be improved and transformed into sustainable material management, ensuring prudent, efficient, and rational utilization of natural resources, promoting the circular economy’s principles. Proper waste management shall follow a hierarchy or order of priority, as shown in the schematic pyramid in Figure 1, which consists of prevention, preparing for re-use, recycling, other recovery (energy), and final disposal environmentally appropriate (Brasil, 2010; European Union, 2008b; Portugal, 2011).
Figure 1. Waste hierarchy (European Union, 2008b).
Waste prevention is considered the most environmentally preferred strategy because it consists of non-production and reduced production, reducing waste at the source (European Union, 2008b). It can be done by buying in bulk, reducing packaging,
or redesigning products. Waste prevention may save natural resources, conserve energy, and reduce other types of pollution (e.g., air and water pollution).
Preparing for re-use consists of re-use of products or components that are not waste for the same purpose they will be conceived, and it is also a source reduction (European Union, 2008b). It can be done by re-using plastic bags, jars, containers, etc., and donating old clothes and furniture. Re-use also may save natural resources, conserve energy, and reduce other types of pollution (e.g., air and water pollution).
Recycling consists of recovery operations by which waste is reprocessed into products, materials, or substances, whether for the original or other purposes (European Union, 2008b). It also includes the recovery of organic matter (e.g., composting and producing carbonaceous materials from biomass). Recycling helps reduce the overall amount of waste sent for disposal and conserve natural resources by replacing the need for virgin materials (US EPA, 2009a).
Other recovery consists mostly of energy recovery (European Union, 2008b). It includes converting materials into heat, electricity, or fuel through various processes, including combustion, gasification, pyrolization, anaerobic digestion, and landfill gas recovery (US EPA, 2009a).
Final disposal consists of operations that are not recovery, even when the process has a secondary effect of substances or energy reclamation. The most common form of proper waste disposal is landfill. Methane gas, a byproduct of decomposing waste, can be collected and used as fuel to generate electricity. Another residue of decomposing waste is the leachate, which needs to be collected and treated (US EPA, 2009a).
2.2 Carbonaceous adsorbents
2.2.1 Carbonaceous materials
A variety of light-weight, relatively low-cost, and environmentally friendly materials with various microtextures (powders, fibers, fabrics, foams, glassy carbon, and composites) contain carbon (Kurzweil, 2009). With multiple chemical bonding possibilities, carbon can be found in many allotropes. Diamond, graphite, nanotubes, and fullerenes are among the carbon-based materials (Nudrat et al., 2018).
AC, pyrochar, hydrochar, carbon black (furnace soot, thermal black), carbon fibers (polyacrylonitrile, phenol resin, pitch), glassy carbon (polymer-based), and carbon foam (nanomaterials, polymer-based) are examples of various materials composed of carbon. Most carbon materials’ key features are high surface area, tailored pore geometry, pore size distribution, wettability, and conductivity (Kurzweil, 2009).
Carbonaceous materials have been widely used for capacitor electrodes in aqueous and aprotic solutions. Powdered graphitic materials usually conduct better than powdered amorphous carbon (e.g., activated carbons, biochar). Porous carbonaceous materials have been widely used in catalysis, energy storage (supercapacitors and Li-ion batteries), energy production (electrocatalysts or electrocatalyst support for fuel cells), gas storage (water (H2O), methane (CH4), carbon dioxide (CO2)) and the removal of contaminants (e.g., heavy metals, gaseous pollutants, dyes) (Sevilla & Fuertes, 2016).
2.2.2 Adsorbent carbonaceous materials
According to IUPAC (2015), adsorption is defined as the enrichment of molecules, atoms, or ions in an interface’s vicinity. In gas-solid systems, the interface material is a solid surface, called adsorbent, and the adsorption takes place in its surface, outside the solid structure. This general phenomenon occurs whenever an adsorbable gas (the adsorptive) is brought into contact with the surface of a solid (the adsorbent) and adsorbs in its surface (becoming the adsorbate). The adsorption space is the area occupied by the adsorbate. Adsorption can be physical (physisorption) or chemical (chemisorption) (Thommes et al., 2015). Physisorption is a physical adsorption attraction of an adsorbate to a surface, the outer surface, and the inner pore surface of an adsorbent by physical forces (Van der Waals forces) (CEN, 2014). Differently, in chemisorption, the adsorbate is trapped on the adsorbent surface due to the intermolecular forces involved in formatting chemical bonds (CEN, 2014; Thommes et al., 2015).
In the context of physisorption, the pores are defined according to their size as follows (Porada et al., 2013; Thommes et al., 2015):
(i) Macropores are the ones with widths greater than about 50 nm; (ii) Mesopores are the ones with widths between 2 nm and 50 nm; (iii) Micropores are the ones with widths less than about 2 nm;
A carbonaceous adsorbent is represented in Figure 2, showing its amorphous form, the porous structure with the adsorbate on it, and its aromatic clusters.
Figure 2. Representation of a carbonaceous adsorbent: (a) amorphous form; (b) porous structure with adsorbate; (c) aromatic clusters.
Based on Celzard et al. (2007).
Adsorbents can be prepared by thermal decomposition of the material, eliminating non-carbon species and producing a fixed carbon mass with a rudimentary pore structure composed of fine and closed pores. The adsorbent can be activated via chemical or physical means to enlarge the pores’ diameters and create new pores (Hu et al., 2001).
2.2.2.1 Bioadsorbents
Nearly all biological materials (e.g., microbial cells, plant and animal biomass, organic waste sludge) have the capacity of pollutant removal/recovery (Safarik et al., 2018). When dead biomass is used in the adsorption of pollutants, they are called bioadsorbents (Fomina et al., 2014). Biosorption is a physicochemical process in which low-cost raw biomass acts as adsorbent (Safarik et al., 2018). Functional groups located on the surface of bioadsorbents play an essential role in the biosorption of pollutants due to their interaction with target pollutants (Safarik et al., 2018). Among the proper handling of exhausted bioadsorbent after the adsorption process is its regeneration and re-use in subsequent biosorption cycles; also, use it as a precursor to producing pyrochars by pyrolysis (Baldikova et al., 2019).
2.2.2.2 Pyrochars
Thermo-chemical processes, such as pyrolysis, are applied to produce char, oil, or gaseous product from biomass. Pyrolysis consists of the organic raw material’s carbonization in an inert atmosphere (Yek et al., 2019). The thermal treatment dehydrates and devolatilizes the biomass during carbonization, resulting in a remaining solid char with high-carbon content, high porosity, large surface area, and high pore volumes, usually called pyrochar (Ioannidou et al., 2007; Lam et al., 2018; Lehmann et al., 2009; Mohamed et al., 2010).
The key parameters controlling the pyrochar properties during the pyrolysis process are temperature, followed by pyrolysis heating rate, nitrogen flow rate (used as the carbonizing agent), pyrolysis residence time, and feedstock type (Ahmad et al., 2014; Chen et al., 2017; Ioannidou et al., 2007; Mohamed et al., 2010).
Pyrolysis processing of biomass enlarges the crystallites and makes them more ordered, an effect that increases under high temperatures (Lehmann et al., 2009). As reported by Chen et al. (2017), the biomass is converted into a “3D network of benzene rings” with plenty of functional groups at temperatures below 500 o C during the pyrolysis process. At temperatures between 500o C and 700o C, it is transformed into a “2D structure of fused rings” with abundant porosity. At temperatures higher than 700 o C, it may transit into a “graphite microcrystalline structure”. The surface area of the biomass char dramatically increases from 400 to 900 o C. Figure 3 presents a schematic of pyrochar structure development under many temperature ranges.
Figure 3. Schematic of pyrochar structure development under many temperature ranges (Lehmann et al., 2009).
Pyrochar has been used as a soil amendment (to increase soil health and productivity sustainably and a tool for atmospheric carbon dioxide sequestration in soils), biofuel, catalytic support, and adsorbent (Alhashimi & Aktas, 2017; Daramola et al., 2020; Lehmann et al., 2009). It has received recognition in diverse applications in recent years due to its adsorption properties (Alhashimi et al., 2017). Pyrochars obtained in relatively high pyrolysis temperatures usually present high surface area, good microporosity, and hydrophobicity, potentially useful in the sorption of organic contaminants. In contrast, pyrochars that are effective in the sorption of inorganic/polar organic contaminants are usually obtained in relatively low pyrolysis temperatures (<= 500 ºC), presenting more oxygen-containing functional groups, electrostatic attraction, and precipitation (Ahmad et al., 2014).
As reported by Alhashimi & Aktas (2017), it is essential to investigate the environmental and economic impact perspective of pyrochar compared to alternative materials such as AC, presented in detail below. The same authors reported that pyrochars have lower environmental impacts than activated carbons. If engineered correctly for the specific application, they could be as efficient as activated carbon and less expensive. Nevertheless, significant gases are released during the pyrolysis process. In the case of pyrolysis of OS, Blanco López et al. (2002) mentioned CO, CO2, CH4, ethylene, ethane, and hydrogen as gases produced during this process, among which CO and CO2 are the main ones.
2.2.2.3 Hydrochars
The hydrochar is a material obtained by HTC. It is another thermochemical process, which uses water, heat (range of 150 to 350 oC), and high pressure to convert biomass into carbonaceous materials through fractionation of the feedstock (Daramola et al., 2020; Jain et al., 2016; Lehmann et al., 2009; Ok et al., 2016). This process is considered a promising waste conversion technique by converting waste into value-added products. It presents the advantages to allow the use of high moisture-containing feedstock without requiring a pre-drying step and to do not generate any hazardous chemical waste or byproducts when performed only with water (Bruckman, 2016; Ok et al., 2016). HTC and pyrolysis are two of the most frequently used processes to prepare carbonaceous materials, with high adsorption capacity, from agriculture residues (Ok et al., 2016).
In HTC, due to temperature, steam is formed, and the pressure rises, leading to a thermo-chemical transformation of biomass (Daramola et al., 2020). Water acts as a solvent and a catalyst facilitating efficient hydrolysis and the partition of the lignocellulosic material (Jain et al., 2016). The hemicellulose content in biomass partly undergoes hydrolysis at lower temperatures and results in the formation of hydrochar through polymerization (water solubility homogenous reaction) (Jain et al., 2016).
HTC method has received growing attention due to its simplicity and ability to deliver hydrochar with many oxygenated functional groups (OFGs) (Jain et al., 2016; Ok et al., 2016). It is considered more environmentally sound because it usually does not generate hazardous outputs, as does dry pyrolysis. Besides, HTC requires less energy, implying economic benefits as well. Figure 4 illustrates the formation of OFGs by HTC.
2.2.2.4 Activated carbons
According to the IUPAC, AC is “a porous carbon material, a char which has been subjected to reaction with gases, sometimes with the addition of chemicals, before, during or after carbonization in order to increase its adsorptive properties” (IUPAC, 1997). The U.S Environmental Protection Agency describes AC as “a highly adsorbent form of carbon used to remove odors and toxic substances from liquid or gaseous emissions…” (US EPA, 2009b). AC consists of macrostructures formed by flat aromatic sheets, broken in places by slit-formed pores, and cross-linked amorphous carbon that defines cylindrical pores by its accidental orientation (Kurzweil, 2009; Shen et al., 2018). The main distinctions between pyrochar and AC are that activation is not performed during pyrochar production while it is crucial for AC production; also, the processes’ temperatures are usually different (Zhang et al., 2017).
AC is used to treat liquids and gases, and it generally has a large adsorption capacity, preferably for small molecules, because of its high pore volume and surface area. The adsorption of compounds by AC is a complex process that depends on a wide range of variables, including adsorbent properties, the nature of the adsorbate, operating conditions (relative humidity, temperature, pressure, and volumetric flow rate), and the presence of adsorption competition (Le-Minh et al., 2018).
An assortment of ACs having distinct porosity can be obtained by controlling activation and carbonization processes. This carbonaceous material is used generally in granular and powdered forms but can also be found in textile form prepared by controlled carbonization and activation of carbon fiber textiles (IUPAC, 1997).
According to Mohd Din et al. (2009), biomass can be converted into AC via chemical, physical, or physiochemical (a combination of the previous methods) activation.
The physical activation methods mostly involve carbonization of the biomass at temperatures below 700 °C, followed by controlled gasification of the char at higher temperatures in a stream of oxidizing with the activating agents steam (CO2, air, NH3, O2, or any mixture of these gases), without the presence of a chemical catalyst (Alslaibi et al., 2013; Chen et al., 2017; Hu et al., 2001). In this type of activation, the raw material is carbonized at first, then volatile compounds are removed, and oxidation sites are created.
The process results in increased aromatic cross-linked sheets, and carbon layers are removed by controlled oxidation. The activation process progress depends on the oxygen added to the steam. High temperatures, long residence times, and favorable oxidizing conditions result in larger micropores and small mesopores (Kurzweil, 2009).
The chemical activation is performed by the chemical treatment of the lignocellulosic material, in which the cellulose structures are destroyed, followed by carbonization and aromatization of the carbon skeleton (Kurzweil, 2009). At the beginning of the process, a chemical agent is added, followed by heat treatment, generally in the 450-900 oC range, of the impregnated material under an inert atmosphere to form the final porous structure known as AC (Mohamed et al., 2010; Sevilla & Mokaya, 2014). The activating agents of chemical activation consist of acids (mostly phosphoric acid, H2SO4, and nitric acid); alkalis (mainly potassium hydroxide (KOH), sodium hydroxide); and salts (mostly zinc chloride (ZnCl2), magnesium chloride, potassium carbonate) (Chen et al., 2017; Mohamed et al., 2010; Sevilla et al., 2014). Finally, the product is washed to remove and recover the excess of the activation agent (Kurzweil, 2009). Chemical activation may retain an abundant distribution of surface functional groups originated from a precursor, which could be effective for polar pollutants, such as NH3 (Zheng et al., 2016).
The main advantages of chemical activation over physical activation are: (i) relatively low energy cost due to lower pyrolysis temperatures; (ii) much higher carbon yield is obtained; (iii) adsorbents with a very high surface area can be produced, and (iv) the microporosity can be well developed, controlled and tailored to be narrowly distributed (Hu et al., 2001; Sevilla et al., 2014).
Hydrochars are frequently used as precursors of ACs, as shown the Figure 5. As described in the previous topic, HTC results in efficient hydrolysis and dehydration of biomass and bestows the hydrochar with high OFGs content, making it a suitable precursor to produce chemically activated carbon.
Figure 5. Preparation of AC by HTC, followed by chemical activation (Jain et al., 2016).
As reported by Yang & Lua (2003), activated carbons’ chemical nature influences its adsorptive, electrochemical, catalytic, and other properties. Predominantly, activated carbons with acidic surface chemical properties are favorable for alkaline gas adsorption, while activated carbons with basic surface chemical properties are suitable for acidic gas adsorption.
Physiochemical activation mainly happened at high temperatures in the presence of dehydrating agents (e.g., KOH, ZnCl2, H2SO4) and of oxidizing agents such as CO2/steam to provide further gasification effect (Alslaibi et al., 2013; Din et al., 2009).
2.2.3 Adsorbents derived from biomass
The solid biomass is available in different forms, with variable moisture contents and chemical elements, including agricultural and forestry residues, biological materials byproducts, wood, organic parts of municipal, and sludge waste (Sansaniwal et al., 2017). The various agricultural biomass wastes are among the most abundant, accessible, and renewable resources used to produce carbonaceous adsorbents, such as bioadsorbents, pyrochars, hydrochars, and ACs (Javidi Alsadi & Esfandiari, 2019).
The production of adsorbents using lignocellulosic materials as precursors has attracted much attention due to the high cost of producing adsorbents from coal, not to mention it is a non-renewable resource. Practically all lignocellulosic materials can be used as feedstock to produce applicable adsorbents. The use of a suitable precursor is mainly conditioned by its availability and cost, although it also depends on the manufactured carbon’s particular applications and the type of installation available. The
